The long-term goal of this proposal is to understand the biological roles of DNA helicase and helicase-related genes in recognizing and repairing spontaneous damage that occurs during the mitotic cell cycle. Mitotic DNA damage is dangerous to DNA replication, and can result in stalled or collapsed replication forks. The DNA damage is processed by lesion bypass pathways or homologous recombination, depending on the type of DNA damage. Homologous recombination (HR) must be tightly regulated to avoid genomic rearrangements. Defects in recombination, particularly from helicase and helicase-like genes, can result in blocked intermediates that trigger a cell cycle checkpoint. We will investigate the roles of helicase and helicase-like genes in repair and recombination, studying the control over with which pathway is used to repair damage during replication. The Srs2 DNA helicase and the SWI/SNF-like proteins Rad54, Rdh54 and Tid4 act against Rad51, an essential factor in homologous recombination. The Srs2 DNA helicase can regulate the type of repair pathway by disrupting a Rad51 filament, one of the first intermediates in HR. We will investigate how the balance between HR and postreplication repair (PRR) is achieved through the balance between the Srs2 helicase and Rad51 protein, and whether replication components have a role in this balance. Rad54, Rdh54 and Tid4 remove Rad51 from double stranded DNA results in a cell cycle checkpoint arrest and increased DNA rearrangements. The specific aims are to understand why the nonessential Srs2 DNA helicase becomes essential in sister chromatid cohesion defective strains and why excess Rad51 protein is lethal, and the roles of the SWI/SNF-like proteins Rdh54 and Tid4 in maintenance of genomic stability. These goals will be achieved by genetic studies, examination of the consequences of Rad51 overexpression, microscopic examination of cells for markers of DNA damage, studies on the role of sister chromatid cohesion recombination repair. DNA repair mechanisms are essential for mutation avoidance and stability of the genetic information. When the repair mechanisms are defective, mutations and chromosome changes occur. These reveal normally silent mutations, which now drive cells to uncontrolled growth associated with human cancers.